Sealing system for the outlet of a plastic-lined compressed gas cylinder

ABSTRACT

A sealing system for an outlet of a plastic-lined cylinder has a plastic liner outlet extending into a bore of a boss. An insert is engageable with the bore, forming a primary seal between the insert and portion of the liner outlet. A tapered compression surface of the insert engages a tapered bore portion of the liner outlet adjacent the primary seal forming a tapered interface and a secondary seal. An axial position in the boss, and dimensional integrity, of the liner outlet is maintained by engaging an annular distal end of the liner outlet in the bore by a stop. The stop can be formed on the insert.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefits under 35 U.S.C 119(e) of U.S.Provisional Application Ser. No. 61/308,736, filed Feb. 26, 2010 andU.S. Provisional Application Ser. No. 61/308,751, filed Feb. 26, 2010,which are incorporated fully herein by reference.

FIELD OF THE INVENTION

The present invention relates to fibre-wrapped, plastic-lined cylinders,particularly to sealing systems within an end fitting of an outlet bossof the cylinder, and more particularly to anti-extrusion and anti-creepmeasures at the sealing interface.

BACKGROUND OF THE INVENTION

Fuel cylinders for liquefied natural gas (LNG), liquefied petroleum gas(LPG), and particularly for hydrogen gas (H2) are ideally as light aspossible. Cylinder structures used to maintain the high pressures andremain lightweight include use of aluminum cylinders or liners woundwith carbon fibres and plastic liners or bladders similarly wrapped incarbon fibres. The carbon fibre wraps provide the necessary structuralintegrity where less structurally-capable, yet low permeability,fuel-retaining liners are used.

Cylinder structures capable of containing high pressures utilizehemispherical or polar heads. Whether lightweight metal cylinders orplastic liners are used, one or more outlets are presently formed ofmetal. For example, a polar head of a fibre-wrapped, plastic linedvessel can be fit with a metal boss. Usually, the metal boss isintegrated with the liner prior to wrapping with carbon fibres. The bosscan have a partial polar shoulder which is over-wrapped as well with thecarbon fibres. The boss is threaded and fit with an “end fitting” orinsert—including a plug, valve or pressure regulator.

The boss itself is needed for installation of an end fitting or insertsuch as a valve or regulator. It is known that the conventional threadedconnections between boss and insert are not adequate to block a leakpath of the pressurized fluid within, and particularly challenging withlow molecular weight gases.

Current versions of lightweight fuel cylinders typically comprise aplastic liner fit with a metal boss and then wrapped with carbon fibre.Due to the introduction of plastic liners, there is now a new problemintroduced with sealing of the boss at the entrance to the cylinder.There is a problematic interface between the liner and the polar endports used to access the interior of the cylinder. The interface of theplastic liner and the metal boss has the potential to leak.

Some manufacturers go to great lengths of providing a seal interfacebetween the boss and the liner by leaving the bore of the boss free ofany liner components and accepting conventional fittings. An example isa tongue and groove form of interface as set forth in U.S. Pat. No.6,227,402 to Shimojima et al. wherein the plastic liner is integratedinto annular grooves in the boss. Other forms where a boss embedded inthe liner itself are set forth in U.S. Pat. No. 5,253,778 to Sirosh,U.S. Pat. No. 5,518,141 to Newhouse et al. and U.S. Pat. No. 7,549,555to Suzuki et al. When a leak does occur, the cylinder cannot be repairedand is scrap.

A characteristic of plastic is a higher rate of creep under sustainedloading. In this case, creep is exhibited at the seal interface of theliner and the insert. Seals typically comprise an elastomeric sealelement compressed against rigid seat. With the introduction of plasticliners, the rigid seat is replaced with the plastic material. Over time,plastic tends to slowly move away from a sealing engagement with theseal element and pressurized fluid then can escape thereby.

It is known in the prior art to introduce a liner outlet into the boreof the boss. Accordingly, there has been an attempt to provide a sealbetween the plastic liner within the boss and the insert. In U.S. Pat.No. 5,938,209 to Sirosh et al. and U.S. Pat. No. 6,186,356 to Berkley etal., an O-ring is sandwiched axially between an annular end face of theliner and end face of the insert. In another form, as set forth inpublished US Application 2009/0071930 to Sato et al., an O-ring islocated between the liner and the boss. Again, should a leak occur, thecylinder cannot be repaired and is scrap.

Other prior art arrangements include placing an O-ring circumferentiallyin an annular groove formed in the insert, the insert and O-ring portionprotruding into and sealing against a cylindrical throat of the liner.

Other factors contributing to seal leakage include differential thermalexpansion of the differing materials. The insert is usually aluminum orstainless steel which has a lower coefficient of thermal expansion thanplastic which can also cause issues at the interface.

Accordingly, a new sealing system would overcome the deficienciesexperienced by the prior art.

SUMMARY OF THE INVENTION

Embodiments described herein are directed to a sealing system formedbetween an outlet of a plastic liner extending into a bore of a boss.The liner outlet and the bore of the boss form a profiled bore. Aninsert, engageable with the profiled bore, forms a profiled surface forsealing with profiled bore. The liner outlet is retained along the boreof the boss for dimensional stability. The sealing system enablessealing using an annular sealing element and avoids extrusion of theseal element therebetween. Other embodiments provide secondary sealingusing compressive interference of a tapered interface between theprofiled bore and profiled surface. Other embodiments providecompression of any assembly clearance to obviate extrusion andoptionally to preload the liner outlet for minimizing the effects ofcreep.

Accordingly in one broad aspect a sealing system for an outlet of aplastic-lined cylinder for compressed gas is provided. The plastic-linedcylinder comprises a plastic liner and a boss. The boss has a bore foraccessing the cylinder. The sealing system comprises a liner outlet ofthe plastic liner extending axially into the bore of the boss to form aprofiled bore. The profiled bore comprises a liner section and a bosssection. The liner section comprises a cylindrical, sealing boreportion, a tapered bore portion, and an annular distal end having anaxial position in the boss's bore. The boss section comprises aninsert-securing bore portion. The sealing system also comprises aninsert having a profiled surface. The insert is engageable with theprofiled bore for sealing the profiled bore. The insert's profiledsurface comprises an annular seal element, a tapered compressionsurface, and a bore-securing surface. The sealing system furthercomprises a retaining shoulder. When the bore-securing surface axiallyengages the insert-securing bore portion, the insert's profiled surfaceengages the profiled bore. The seal element corresponds and seals withthe sealing bore portion to form an annular primary seal. The retainingshoulder engages the annular distal end for retaining the annular distalend at the axial position in the boss's profiled bore. The taperedcompression surface corresponds and engages with the tapered boreportion to form an annular tapered interface and compresses the lineroutlet to form a tapered secondary seal.

In an embodiment, the tapered interface is adjacent the primary seal forclosing any annular assembly clearance. In another embodiment the insertis provided with a leakage path downstream of the primary seal to bypassthe tapered interface.

Accordingly in another broad aspect a method for sealing theplastic-lined cylinder is provided. The method comprises axiallyextending a liner outlet of the plastic liner into the bore of the bossto form a profiled bore. An insert having a profiled surface is engagedinto the profiled bore for forming an annular primary seal and a taperedsecondary seal with the profiled bore liner outlet. The primary andsecondary seals are formed by first axially inserting the insert intothe profiled bore and engaging a bore-securing surface on the insertwith an insert-securing bore portion of the profiled bore for securingthe insert to the boss. The primary seal is formed by engaging anannular seal element of the profiled surface insert with a cylindricalsealing bore portion of the profiled bore. The secondary seal is formedby compressing a tapered compression surface of the profiled surfacewith a tapered bore portion of the profiled bore for forming an annulartapered interface. The tapered interface forms the secondary seal. Anaxial position of an annular distal end of the liner outlet ismaintained in the boss's bore by retaining the annular distal end of theliner outlet against a stop.

Accordingly in yet another broad aspect a method for servicing a sealingsystem of an outlet of a plastic-lined compressed gas cylinder isprovided. The cylinder comprises a plastic liner and a boss. The bosshas a bore for accessing the cylinder which is normally sealed with aninsert. The method comprises disengaging the insert from the outlet ofthe cylinder for exposing a profiled bore of a liner outlet extendingaxially into the bore of the boss. An annular seal element located aboutthe insert is replaced. The seal element normally sealably engages theprofiled bore. At least one sealing surface located on the profiled boreis refurbished.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a partial cross-sectional view of a plastic-lined,fiber-wrapped cylinder according to one embodiment, the polar boss beingfit with a plug-type of insert and the insert having a retainingshoulder;

FIG. 1B is an exploded view of the polar boss of FIG. 1 fit with a flowthrough-type of insert, the insert shown prior to engagement with theboss and the liner outlet shown prior to engagement with the boss;

FIG. 2 is an enlarged view of the polar boss of FIG. 1 fit with a flowthrough-type of insert;

FIG. 3 is an enlarged view of the polar boss of FIG. 1 fit with theplug-type of insert;

FIG. 4A is an enlarged view of the liner, an annular tapered interface,an annular seal element and backer ring common to both embodiments ofthe inserts of FIGS. 2 and 3;

FIGS. 4B and 4C are enlarged, staged views of the seal element at thetapered interface of FIG. 4A, FIG. 4B illustrating the seal elementbefore engaging the liner and FIG. 4C illustrating the seal elementafter engagement with the liner;

FIG. 5 is an isometric view of one embodiment of a boss;

FIG. 6 is a partial cross-sectional view of the liner and liner outletcompatible with a boss such as that of FIGS. 1 through 4 and 5;

FIGS. 7A and 7B are side and cross-sectional views respectively of aform of polar boss;

FIG. 8 is an isometric view of the flow through-type insert according toFIG. 2;

FIG. 9 is an isometric view of the plug-type insert according to FIGS. 1and 3;

FIGS. 10A and 10B are side and cross-sectional views respectively of theflow through-type insert according to FIG. 2;

FIGS. 11A and 11B are side and cross-sectional views respectively of theplug-type insert according to FIGS. 1 and 3;

FIG. 12 is a cross-sectional view of another embodiment of the boss withthe insert installed therein, the boss having the retaining shoulder andthe insert having the seal element;

FIG. 13 is a cross-sectional view of a further embodiment of the bossand the insert installed therein, the boss having the retaining shoulderand the insert having the seal element and the backer ring;

FIG. 14 is a partial cross-sectional view of the insert's profiledsurface and the boss's profiled bore according to FIG. 13;

FIG. 15 is a partial cross-sectional view of the profiled surface andprofiled bore according to FIG. 13 being fit with a backpressureanti-extrusion insert;

FIG. 16 is a partial cross-sectional view of the profiled surface andprofiled bore according to FIG. 13; and

FIGS. 17A and 17B are cross-sectional exploded views of the threadedassembly of the liner outlet (FIG. 17A) to the boss (FIG. 17B).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Herein, embodiments are directed to a sealing system for an outlet of aplastic lined cylinder for compressed gas. The cylinder comprises aplastic liner having a liner outlet and a boss coupled with the lineroutlet. The boss has a bore for accessing the cylinder. For storage ofcompressed gas, the liner is supported against bursting using anoverlying structure such as a carbon fibre wrap. The plastic liner andboss are wound with carbon fibres to provide the necessary structuralintegrity. The boss and the liner are integrated through afibre-wrapping to form the cylinder.

The liner outlet extends axially into a bore of the boss to form aprofiled bore. The profiled bore is sealed by an insert engageable withthe profiled bore. The insert has a corresponding profiled surface. Whenthe insert is engaged with the boss, at least an annular primary seal isformed between an annular seal element of the profiled surface and theprofiled bore. In one embodiment, an annular, tapered, secondary seal isalso formed between the profiled surface and the profiled bore. Thesecondary seal can be formed by interference and compression at anannular tapered interface between the profiled surface and profiledbore. The liner outlet is restrained in the boss by retaining an annulardistal end of the liner outlet against a stop.

In one embodiment, the stop is a retaining shoulder provided on theinsert. In another embodiment, the stop is a retaining shoulder providedin the boss. In another embodiment, compression of the liner outlet atthe tapered interface can also close any annular assembly clearance toblock extrusion of the seal element at high pressures. Compression ofthe liner material can also minimize creep of the sealing surfacesotherwise susceptible to sustained sealing element and pressure loads.Further, the arrangements disclosed herein enable methods forrefurbishing the sealing system. Should a leak develop over time, theliner or insert interfaces can be accessed for servicing and to quicklyplace the cylinder back into service.

FIGS. 1 to 11B illustrate an embodiment of the sealing system whereinthe insert is provided with a retaining shoulder. The embodiments shownin FIGS. 1 to 11B are suitable and tested for use with the storage ofconventional gas such as compressed natural gas (CNG) at pressures ofabout 250 bar.

With reference to FIGS. 1A, 1B, 2 and 5, the polar head 1 of aplastic-lined, fuel cylinder 2 is fit with a rigid boss 3, such as ametal boss, having a bore 4 therethrough for accessing the cylinder. Theboss 3 has a flare 3 a at a vessel end and a fuel opening 3 b (FIG. 5)at an outer end for accessing the bore 4 of the boss and the interior ofthe cylinder or vessel. An outlet 5 a of a liner 5 extends into the bore4 of the boss 3 and is coupled to the boss 3. In an embodiment, theouter surface of the liner outlet is threaded 5 b for coupling with aninternal, threaded portion 3 c of the boss. Alone or in addition tothreaded coupling, an adhesive can also be used between the liner outlet5 a and the boss 3 for coupling the liner outlet 5 a to the boss 3. Theboss 3 and liner 5 are wrapped with a carbon fibre wrap 7, sandwichingthe flare 3 a of the boss within the fibre-wrap and therefore integratedinto the polar head 1 of the cylinder 2. The bore 4 of the boss 3 issealed by an insert 9. The insert 9 is releasably engageable with theboss 3.

As shown in FIG. 1B, the insert 9 has a generally cylindrical body whichincludes an external, threaded portion 9 a which engages with theinternal, threaded portion 3 c of the boss 3 for releasably coupling theinsert 9 with the boss 3. Sealing of the vessel's pressurized fuelcontents is performed at the interface of the insert 9 and the lineroutlet 5 a.

Having reference to FIG. 4A, a sealing system between the insert 9 andthe liner outlet 5 a comprises a profiled bore 4 a (best illustrated inFIG. 1B) formed by the liner outlet 5 a extending into the bore 4 of theboss. The insert 9 has a complementary profiled surface 4 b (bestillustrated in FIG. 1B) for engagement with the profiled bore 4 a.Axially spaced, as referenced from the inside of the vessel or cylinder2, the profiled bore 4 a comprises a liner section LS (best illustratedin FIG. 1A) and a boss section BS (best illustrated in FIG. 1A). Theliner section LS comprises a cylindrical, sealing bore portion 12, atapered bore portion 13, and an annular distal end 14. The annulardistal end 14 has an axial position in the bore 4. The boss section BScomprises an insert-securing bore portion 15. Preferably, theinsert-securing portion 15 is the internal threaded portion 3 c of theboss. The insert's profiled surface 4 b comprises an annular sealelement 17 fit to an annular recess 17 a formed circumferentially abouta cylindrical plug portion 18 of the insert 9 for corresponding andsealing with the sealing bore portion 12 during insertion of the insert9. A typical annular seal element 17 is a form of O-ring. One suitablematerial for the O-ring, for many compressed gases including CNG, isnitrile 700LT.

In this embodiment, the profiled surface 4 b of the insert 9 furthercomprises a tapered compression surface 19, a retaining shoulder 20 anda bore-securing surface 21. Preferably, the bore-securing surface 21 isthe external threaded portion 9 a of the insert. The insert's profiledsurface 4 b engages the profiled bore 4 a when the bore-securing surface21 axially engages the insert-securing portion 15.

In operation, threaded insertion of the insert 9 locates the insert'sseal element 17 with the liner's sealing bore portion 12 to form anannular primary seal between the insert 9 and the liner outlet 5 a.Further, the insert's tapered compression surface 19 axially engages theliner section's tapered bore portion 13 at an annular tapered interface4 c to form an annular, tapered secondary seal between the insert 9 andthe liner outlet 5 a. The tapered interface 4 c is a truncated frustumof a right circular cone. The insert's retaining shoulder 20 engages andaxially retains the distal end 14 of the liner outlet 5 a formaintaining the axial position of the annular distal end 14 at the axialposition in the bore 4 of the boss 3.

As shown in FIGS. 4A and 4C, the seal element 17 is fit within thecylindrical plug portion 18 of the insert 9. The cylindrical plugportion 18 engages the cylindrical sealing bore portion 12 of the linersection LS of the profiled bore 4 a forming a small annular assemblyclearance or gap 29 (FIG. 4C) therebetween (not distinguishable at theresolution of the drawings). The assembly clearance enables the sealelement 17 to enter the cylindrical sealing bore portion 12 duringassembly. However, this assembly clearance can also introduce challengesat higher pressures. The annular seal element 17, under a combination offactors including the seal material properties, the dimensions of theassembly clearance and the pressure, would be subject or vulnerable toextrusion out of the annular recess 17 a and into the assemblyclearance.

As shown in FIG. 4B, the annular seal element 17 is elastomeric and inthe uncompressed, free state, has a first relaxed cross-section, shownin FIG. 4B as a circular cross-section of an O-ring embodiment. As shownin FIG. 4C, when the annular seal element 17 engages the sealing boreportion 12, it is compressed into the annular recess 17 a, substantiallyassuming the cross-sectional shape of the recess 17 a. A backer ring 22can also be provided in the annular recess 17 a. The backer ring 22 islocated in the annular recess 17 a between the annular seal element 17and a wall 17 b of the annular recess 17 a between the tapered interface4 c and the annular seal element. The backer ring 22 can include anannular and concave alignment recess 22 a into which the annular seal 17compresses. Backer rings can assist in resisting movement of the annularseal element 17 during insert installation and when compressed, expandradially to reduce risk of extrusion. The compressive action of theannular seal element 17 into concave alignment recess 22 a can also aidin the radial displacement of the backer ring 22 within the annularrecess 17 a and blockage of the assembly clearance. Suitable backer ringmaterial includes 90 durometer nitrile.

Also, it is found that the compression can also form a directinsert-to-liner seal or the secondary seal at the tapered interface 4 c.

The liner outlet 5 a is restrained from axial movement by the retainingshoulder 20, such movement including that from axial extrusion or creepunder compressive forces imparted by the tapered compression surface andcylinder pressures.

As a result a simple and reliable sealing system or arrangement isachieved.

In one embodiment of the liner as shown in FIG. 6, the liner 5 is abladder comprising about 5 mm of high density polyethylene (HDPE) whichis suitable for natural gas. The cylindrical sealing surface can beabout 33.3 mm in diameter, 20 mm in axial length and the outer diameterof the liner outlet 5 a is about 42 mm for a liner thickness of about4.35 mm. The tapered compression surface 19 is about 5.4 mm in lengthaxially and has taper of 20 degrees from the axis.

In another embodiment of the liner, the liner 5 could include asupplemental layer of EVOH EVAL F101B for improved resistance topermeability of the fuel gas.

The liner outlet 5 a is axially aligned with the boss 3 to maintain theaxial position of the annular distal end 14 in the boss's profiled bore4 a. The insert 9 is axially aligned with the boss 3 so that theretaining shoulder 20 engages and retains the annular distal end 14 atthe axial position when the tapered compression surface 19 engages thetapered sealing bore portion 13. In one embodiment, axial alignmentbetween the boss 3 and the liner 5 is provided as follows: the boss 3 isprovided with a first annular datum surface 24 a and a second annulardatum surface 24 b (best illustrated in FIGS. 1A and 1B). The insert 9is provided with a terminating outer 24 c. The datum surface 24 a isprovided between the flare 3 a of the boss 3 (which is ultimatelycovered in fibre-wrap) and a surface of the polar head end of the liner5 (preferably machined to correspond to the boss flare 3 a, such as aflat surface perpendicular to the vessel axis. The datum surface 24 b isprovided at the fuel opening 3 b of the boss. The annular distal end 14of the liner outlet 5 a is axially spaced a reference distance from theannular datum surface 24 a. As the distance from the fuel opening 3 b ofthe boss to the flare 3 a of the boss is known, distance of the annulardistal end from the fuel opening 3 b of the boss can be calculated. Thisforms a second reference distance. The retaining shoulder 20 is spacedfrom the outer terminating shoulder 24 c by a distance equal to thesecond reference distance so that when the terminating outer shoulder 24c engages the annular datum surface 24 b, the retaining shoulder 20engages and retains the annular distal end 14 at the axial position whenthe tapered compression surface 19 is engaged with the tapered boreportion 13.

The boss 3 and insert 9 are typically formed of aluminum alloy such asanodized AA6061-T6 ASTM B221. The liner outlet 5 a can be secured by ametal-bonding adhesive to the inside of the boss 3. The adhesive canassist with one or more aspects including securing the liner outlet 5 ato restrain the liner outlet within the boss during assembly and to fillinconsistencies in the mating of the liner and boss for maintainingdimensional stability. A suitable adhesive is a two-component, high peelstrength, metal-bonding adhesive. An example of a suitable adhesiveincludes Loctite™ U05-FL (Trademark of Henkel, Ohio USA).

In an embodiment of the insert 9 as shown in FIGS. 1A, 1B, 2, 8, 10A and10B, the insert 9 includes an additional tube extension 25 which extendsinto the interior of the cylinder 2. During the filling and emptying ofthe cylinder 2, the gas flow is throttled through the insert 9, or afitting installed to the insert 9. Pressurization of the cylinder 2 canresult in localized temperature increases and decompression can resultin temperature decreases. The tube extension 25 can act to shift suchthermal effects to the interior of the cylinder 2 and can also act as astatic build-up accumulator away from the plastic surface. In any event,grounding of the insert 9 to an exterior ground, such s the mountingassemblies can bleed off static build-up.

FIGS. 12 to 17 illustrate an embodiment of the sealing system whereinthe axial position of the annular distal end 14 of the liner outlet 5 ain the bore 4 of the boss 3 is maintained by a retaining shoulder 20 aformed in the boss 3. The embodiments shown in FIGS. 12 to 17 have beenshown suitable for use with the storage of compressed gas at highservice pressures of about 700 bar (with a safety factor of 1.25 orpressures of about 875 bar). The liner 5 illustrated in FIGS. 12 to 17is well suited to contain compressed gas at pressures ranging from about250 bar to about 875 bar and at temperatures ranging from about −40° C.to about 85° C. The compressed gas can be hydrogen, helium or methane.The liner 5 suited for storage of such gases at pressures stated abovecan be a monolayer bladder or a multilayer bladder comprising a EVOHEVAL F101B layer, a BASELL LUPOLEN 4261A (Trademark of LyondellBasellIndustries Holding B.V., Rotterdam Netherlands) layer and a layer ofDUPONT BYNEL 40E529 (Trademark of DuPont, Delaware USA).

The sealing system illustrated in FIGS. 12 to 17 and the sealing systemillustrated in FIGS. 1A to 11B have common elements. The common elementsare the profiled bore 4 a formed by the liner outlet 5 a extendingpartially into the bore 4 of the boss 3 and the insert 9 having aprofiled surface 4 b. The profiled bore 4 a has the cylindrical sealingbore portion 12, the tapered bore portion 13 and the insert-securingportion 15. The profiled surface 4 b of the insert 9 comprises theannular seal element 17 located in the recess 17 a in the cylindricalplug portion 18, the tapered compression surface 19 and thebore-securing surface 21. A difference between the sealing systemdescribed in FIGS. 1A to 11B and the sealing system described in FIGS.12 to 17 is that in the sealing system described in FIGS. 1A to 11B theretaining shoulder 20 is located on the insert 9 and in the sealingsystem described in FIGS. 12 to 17 the retaining shoulder 20 a islocated on the boss 3. The retaining shoulder extends radially into thebore 4 of the boss. Also, in the sealing system described in FIGS. 12 to17, the annular tapered interface 4 c (portion where the tapered boreportion 13 meets the tapered compression surface 19) is substantiallyimmediately adjacent the annular recess 17 a housing the annular sealelement 17. The sealing system described in FIGS. 12 to 17 generallyworks in the same manner as the sealing system described in FIGS. 1A to11B except that the function of the tapered interface 4 c in FIGS. 12 to17 is more directed towards providing compression of the liner outlet 5a around the annular seal element 17. Compression of the liner outlet 5a at the tapered interface 4 c aids in anti-extrusion of the annularseal element 17 from the annular recess 17 a and also resists reactivecreep from around the annular seal element 17. More detailed functioningof the tapered interface 4 c in FIGS. 12 to 17 is explained below.

In the sealing system described in FIGS. 12 to 17, a small annularassembly clearance 29 (FIGS. 12 and 14) is provided between thecylindrical plug portion 18 of the insert 9 and the cylindrical sealingbore portion 12 of the liner section LS to enable the annular sealelement 17 to enter the cylindrical sealing bore portion 12 duringassembly. The annular assembly clearance 29 is not distinguishable atthe resolution of the drawings. At high pressures, the annular sealelement 17 can extrude from the annular recess 17 a into the annularassembly clearance 29. Compression of the liner outlet 5 a at thetapered interface 4 c can act to reduce and close the assembly clearancefor avoiding opportunity for extrusion of the annular seal element 17.The tapered interface 4 c is located downstream fromnormally-pressurized gaseous contents of the vessel 2. The taperedinterface 4 c is caused to be in such intimate interference andcompressive contact as to effectively eliminate the annular assemblyclearance and thus obviate and tendency or opportunity for extrusion. Inan embodiment, the tapered interface 4 c is immediately adjacent theannular recess 17 a, ensuring any assembly clearance is closedimmediately adjacent the annular seal element 17.

The insert's annular seal element 17 imposes a radial sealing load onthe cylindrical sealing bore portion 12. At high pressures, over time,liner creep in response to sustained radial sealing load can lessen thesealing contact and result in leakage past the annular seal element 17.The annular seal element 17 can also extrude from the annular recess 17a into the annular assembly clearance when the sealing contact islessened. This problem can be overcome by sufficiently pre-stressing theliner 5 around the annular seal element 17. Threaded engagement of theinsert 9 compresses the tapered compression surface 19 and compressivelyloads the liner outlet 5 a at the tapered bore portion 13, thecompressive influence including the material of the liner about thecylindrical sealing bore portion 12. The tapered bore portion 13 iscompressed between the insert's tapered compression surface and the boss3 at the tapered interface 4 c. The load imparts sufficient pre-stressin the liner 5 to resist and counteract creep.

A threshold compression or pre-determined amount of pre-stress can becontrolled. In one embodiment, the threshold compression is controlledby monitoring the torque necessary to set the insert 9 into the profiledbore 4 a. In this embodiment, the insert's profiled surface 4 b isthreadably engaged with the profiled bore 4 a. The insert 9 is rotatedto engage the tapered bore portion with the tapered compression surfacefor compressing the liner outlet 5 a around the annular seal element 17.The torque necessary to engage the tapered bore portion with the taperedcompression surface to a threshold compression is monitored. Rotation ofthe insert 9 is stopped when a threshold torque signifies the thresholdcompression has been achieved. As the insert 9 is threaded into theprofiled bore 4 a the tapered surface 19 meets the tapered bore portion13 and the tapered bore portion 13 is compressed. The torque necessaryto rotate the insert 9 increases to the threshold torque. When thetorque reaches the threshold torque, sufficient compression has beenachieved. Further, the retaining shoulder 20 a engages the annulardistal end 14 of the liner 5, firstly as a stop for preventing axialmovement of the liner outlet 5 a to maintain dimensional integrity andoptionally, secondly, for imparting additional compression.

In another embodiment, the threshold compression is controlled bydetermining a corresponding axial threaded insertion and forming theinsert 9 with the outer terminating shoulder 24 c (such as the diametralchange at the hexagonal tool) to engage the boss 3 when the necessarythreaded insertion has been achieved. One can match the datum surface 24on the boss 3 with the terminating shoulder 24 c on the insert 9, theaxial positioning of each of which being calculated to stop the axialinsertion of the insert by “bottoming” of the terminating shoulder 24 cagainst the boss's datum surface 24 b when the threshold compression isachieved. The axially spacing of the elements of the insert 9 and theelements of the boss 3, are dimensionally spaced so as to: locate theO-ring seal 17 in the cylindrical sealing bore portion, engage andcompress liner's tapered bore portion 13, and axially retain the annulardistal end 14 of the liner upon or prior to the insert's outsidecomponents engage or bottom out on the boss's outside face.

With reference to FIG. 15, in the case of an O-ring seal, and over time,high pressure gas such as hydrogen permeates through the O-ring andfills any minor downstream O-ring voidage between the annular sealgroove or recess 17 a, the O-ring 17 and the cylindrical sealing surface12. During rapid decompression of the cylinder 2, trapped, high pressuregas in the annular recess 17 a will be restricted from escaping. If therate of decompression is sufficiently high, then the trapped gas canpush the O-ring 17 back into the cylinder 2, extruding along theparallel, cylindrical surfaces of the liner 5 and insert 9.

A solution is to provide a small decompression leak path from the O-ringvoidage, to bypass the tapered interface, and into the insert-securingportion 15 or threaded section between the boss 3 and the insert 9,which itself is a poor seal.

Accordingly, a conical sleeve 28 is provided as part of the insert 9 andis formed of a compatible insert material, such as metal. Thedecompression leak path formed between the conical sleeve 28 and theinsert 9 extends between the annular seal element 17 and thebore-securing portion 21. The conical sleeve 28 is fit to the insert 9between the annular seal element 17 and the bore-securing portion 21.The conical sleeve 28 has a tapered outside surface 28 a, a cylindricalinside bore 28 b and an upstream end 28 c. The tapered outside surface28 a forms the insert's tapered compression surface 19 and performs thesame compression objectives as described in the previous embodiment whenthe conical sleeve 28 is fit to the insert. The upstream end 28 c formsa wall of the annular recess 17 a housing the annular seal element 17when the conical sleeve 28 is fit to the insert 9. The cylindricalinside bore 28 b which fits to a cylindrical outside surface of theinsert 9 forms a metal-to-metal interface. The metal-to-metal interfaceforms a decompression leak path between the conical sleeve 28 and theinsert 9. The leak path extends from the O-ring voidage, past theretaining shoulder 20 and threaded portion between the insert 9 and theboss 3. The O-ring voidage is downstream of the annular seal element 17.Little of the cylinder's contents should pass the O-ring seal 17.Accordingly, while the metal-to-metal interface is restrictive, there isa sufficient leak path to avoid a high pressure build up and associatedenergy capable to extruding the O-ring 17 on quick or rapiddecompression of the cylinder 2.

Differential thermal expansion of the differing materials at the outletcan be minimized. Differential thermal expansion can occur as theplastic material of the liner outlet 5 a has a higher co-efficient ofthermal expansion (CTE) than that the material of the boss 3. In bothembodiments of the sealing system, radial expansion of the liner outlet5 a due to temperature changes can be minimised by reducing thethickness of liner material in the liner outlet 5 a. In both embodimentsof the sealing system, should a leak develop over time due todeterioration of the O-ring or the sealing surface on the liner outlet 5a or the sealing surface on the insert 9, the liner outlet or insertsealing surfaces can be serviced or repaired. This is possible as theliner outlet 5 a extends into the bore 4 of the boss 3 and can be easilyaccessed for repair or service. As the O-ring is located on the insert 9and since the insert 9 can be disengaged from the boss, the O-ring canalso be easily replaced.

Accordingly, a method for servicing the sealing system of FIGS. 1A to11B and FIGS. 12 to 17 is provided. The method comprises disengaging theinsert 9 engaged with the liner outlet 5 a from the bore 4 of the boss 3for exposing the profiled bore 4 a. The annular seal element 17 isreplaced, if the same requires replacement. The annular seal elementnormally sealably engages with the profiled bore 4 a. The sealingsurfaces located on the liner outlet 5 a namely the tapered bore portion13 and/or the cylindrical sealing bore portion 12 are refurbished. Thesealing surfaces located on the insert 9, namely the tapered compressionsurface 19 and the cylindrical plug portion 18, while robust, could alsobe refurbished. The bore-securing portion 21, the insert securingportion 15, the annular distal end 14 and the retaining shoulder 20, 20a can also be refurbished as necessary during the servicing operation.

1. A sealing system for an outlet of a plastic-lined cylinder forcompressed gas, the plastic-lined cylinder comprising a plastic linerand a boss, the boss having a bore for accessing the cylinder, thesealing system comprising: a liner outlet of the plastic liner extendingaxially into the bore of the boss to form a profiled bore, the profiledbore comprising a liner section and a boss section, the liner sectioncomprising a cylindrical, sealing bore portion, a tapered bore portion,and an annular distal end having an axial position in the boss's bore,and the boss section comprising an insert-securing bore portion; aninsert having a profiled surface and engageable with the profiled borefor sealing the profiled bore, the insert's profiled surface comprisingan annular seal element, a tapered compression surface, and abore-securing surface; and a retaining shoulder; wherein, when thebore-securing surface axially engages the insert-securing bore portion,the insert's profiled surface engages the profiled bore, the sealelement corresponding and sealing with the sealing bore portion to forman annular primary seal, the retaining shoulder engaging the annulardistal end for retaining the annular distal end at the axial position inthe boss's bore, and the tapered compression surface corresponding andengaging with the tapered bore portion for forming an annular taperedinterface and compressing the liner outlet to form a tapered secondaryseal.
 2. The sealing system of claim 1, wherein the retaining shoulderis formed in the insert's profiled surface between the bore-securingsurface and the tapered compression surface.
 3. The sealing system ofclaim 1, wherein the seal element is fit in an annular recess formedcircumferentially about the insert's profiled surface.
 4. The sealingsystem of claim 4, wherein the tapered interface is axially spaced fromthe annular recess.
 5. The sealing system of claim 3, wherein theannular recess further comprises a wall between the seal element and thetapered interface, the system further comprising a backer ring locatedin the annular recess and positioned axially between the seal elementand the wall for resisting extrusion of the seal element into thetapered interface.
 6. The sealing system of claim 1, wherein theinsert-securing bore portion is an internal threaded portion and thebore-securing surface is an external threaded portion.
 7. The sealingsystem of claim 1, wherein the tapered interface is a truncated frustumof a right circular cone.
 8. The sealing system of claim 1, wherein theliner is a monolayer or a multi-layer structure.
 9. The sealing systemof claim 1, wherein the liner outlet is threadably coupled to the boss.10. The sealing system of claim 1, wherein the liner outlet is coupledto the boss by a metal-bonding adhesive material.
 11. The sealing systemof claim 1, wherein the liner is adapted to store the compressed gas atpressures of about 250 bar.
 12. The sealing system of claim 1, whereinthe insert is a flow through type or plug type insert.
 13. The sealingsystem of claim 1, wherein the boss has a first annular datum surface atan outlet of the boss and a second annular datum surface at a flare ofthe boss and the insert has a terminating outer shoulder, the systemfurther comprising: the annular distal end of the liner outlet isaxially spaced a reference distance from the first annular datumsurface; and the retaining shoulder is similarly spaced the referencedistance from the terminating outer shoulder so that when theterminating outer shoulder engages the first annular datum surface, theretaining shoulder engages and retains the annular distal end at theaxial position when the tapered compression surface is engaged with thetapered bore portion.
 14. A method for sealing an outlet of aplastic-lined cylinder for compressed gas, the plastic-lined cylindercomprising a plastic liner and a boss, the boss having a bore foraccessing the cylinder, the method comprising: axially extending a lineroutlet of the plastic liner into the bore of the boss to form a profiledbore; and engaging an insert having a profiled surface into the profiledbore for forming an annular primary seal and an annular secondary sealwith the profiled bore liner outlet by: axially inserting the insertinto the profiled bore and engaging a bore-securing surface on theinsert with a insert-securing bore portion of the profiled bore forsecuring the insert to the boss; engaging an annular seal element of theprofiled surface insert with a cylindrical sealing bore portion of theprofiled bore for forming the primary seal by, compressing a taperedcompression surface of the profiled surface with an tapered bore portionof the profiled bore for forming an annular tapered interface, thetapered interface forming the secondary seal; and maintaining an axialposition of an annular distal end of the liner outlet in the boss's boreby retaining the annular distal end of the liner outlet against a stop.15. The method of claim 14 wherein the maintaining of the axial positionof an annular distal end in the boss further comprises: providing thestop in the insert's profiled surface between the bore-securing surfaceand the tapered compression surface; and engaging the stop against theannular distal end of the liner outlet against the stop.
 16. The sealingsystem of claim 14, wherein the maintaining of the axial position of anannular distal end in the boss's bore comprises, pre-determining theaxial position of the annular distal end of the liner outlet in theboss's bore and axially spaced a reference distance from a first datumsurface on the boss; and pre-determining the axial position of the stopaxially spaced the reference distance from a datum surface on theinsert; and engaging the insert and profiled surface with the profiledbore so that, when the first and second datum surfaces engage, theretaining shoulder engages and retains the annular distal end at theaxial position and the tapered compression surface is engaged andcompresses the tapered bore portion.
 17. A method for servicing asealing system of an outlet of a plastic-lined compressed gas cylinder,the cylinder comprising a plastic liner and a boss, the boss having abore for accessing the cylinder which is normally sealed with an insert,the method comprising: disengaging the insert from the outlet of thecylinder for exposing a profiled bore of a liner outlet extendingaxially into the bore of the boss; replacing an annular seal elementlocated about the insert, the seal element normally sealably engagingthe profiled bore; and refurbishing at least one sealing surface locatedon the profiled bore.
 18. The method of claim 17 wherein therefurbishing further comprises refurbishing a cylindrical sealing boreportion of the profiled bore which normally seals with the seal element.19. The method of claim 17 wherein the profiled bore comprises a sealingbore portion and tapered bore portion, the refurbishing furthercomprises: refurbishing the sealing bore portion which normally sealswith the seal element; and refurbishing the tapered bore portion whichnormally engages a tapered compression surface on the insert.